This lesson covers skills for growing food in containers of variable size.
Some key points:
- A pot or container is anything holding compost – it could even be an old bath; the important thing is good drainage.
- Drainage is to enable water to run away from the bottom, and is not improved by crocks in pots.
- A pot’s measurement, as given in this chapter, is the diameter across its top.
- Beds of compost spread on soil are not ‘containers’ because plant roots go down and soil life moves up, so a bed becomes part of the garden soil, even though it looks like it is sitting op top.
- However beds can be placed and filled on concrete, as long as they are on a sufficient slope that water can drain away from the bottom.
Growing in containers requires more time compared to growing in soil, especially the time taken for regular watering. An informed choice of which vegetables and varieties to grow can give a fair return on this investment of time, and pleasure in the growing.
If you have space for even a small bed, preferably filled with compost rather than soil, you can grow a lot of harvests through one season and more easily than in pots.
Compost
A key requirement for successful growing in pots and containers, even more than in beds, is to use a well-structured compost with a long-term supply of nutrients. I have trialled many composts and find best long-term growth from organic-approved ones.
In genuinely organic composts, the nutrients come from decomposed plant matter and not from added chemical feeds. However, unlike for food and beverage products, the label ‘organic’ on sacks of compost and manure means nothing, unless verified by a certifying symbol from a national body, such as The Soil Association (UK).
- Nutrients from chemical feeds (‘synthetic’, ’nonorganic’) are mostly soluble in water and need careful management.
- Nutrients in organic and homemade composts do not wash out, and release slowly over a long period. In the photos below, see how the lettuce in my homemade compost starts more slowly but then finishes the most strongly.
Lettuce trial
Mottisone lettuce plants were sown in early June and raised in modules. I transplanted them into four pots of different composts in early July and harvested the outer leaves three times altogether.
- The fastest growth was from organic multipurpose compost (Moorland Gold), the second fastest was from well decomposed cow manure, and the third fastest was my homemade compost, in which the lettuce grew strongly for the longest period of time.
- The worst growth was from the green waste compost, from a sack labelled ‘organic farmyard manure’ – it was not at all what it claimed to be and was full of bits of wood. The wood kept sucking nutrients for its own decomposition, hence the very weak growth of lettuce.
This photo of aubergine plants shows how plant leaves look when short of nitrogen, with their leaves turning pale and yellow. They may also show a hue of purple and rose, depending on the plant.
Pea plant trial
In a comparison of pea plants growing in two different composts, I was pleasantly surprised by those in the pure Homeacres compost (see photo below). The proprietary ‘Champions Blend’, in contrast, is ‘formulated for strong growth in pots and containers’. It’s perhaps not so much short of food, as lacking a good balance to enable growth.
- Compost manufacturers rely on laboratory analysis to asses how good their product is, and it seems they do not have time to grow plants in it!
An easy container harvest
Salad leaves give you the fastest, most regular and largest harvests from small amounts of growing medium, particularly when you keep picking the outer leaves and not cutting over the top. In the photos below the plants are growing in 7 cm / 3 in pots.
You can use old mushroom boxes, available for free in many stores, for growing salad and other plants – their drainage and general aeration levels are excellent. Line with just two sheets of newspaper, then pack in as much compost as you can, pushing it down firmly.
- Firm compost gives a larger supply of nutrients and moisture, stability for roots, and results in a longer period of harvest.
For winter growing in boxes, I recommend any type of lettuce, frizzy endive, mustards of all kinds, salad (not wild) rocket, Claytonia, land cress, chervil and coriander. Grow just six plants per box, to make picking easier and the harvest period longer.
These plants tolerate freezing – you can grow them in an unheated greenhouse. However, growth is slow in frosty weather and temperatures below about –12°C / 10°F can damage the plants. In cold weather, plants stay healthier when not too damp. Allow a week or more between watering in midwinter, then more frequently in early spring.
The photos below, showing plants before and after picking, are to illustrate how much you can harvest each time. Pick every two weeks or so, from November to February, then weekly until late April. They can total as much as 900 g / 2 lb of leaves per box.
- Spring plantings in boxes could be wild rocket, peas for shoots, spinach and lettuce.
- In summer you could grow lettuce, frizzy endive, summer purslane, and herbs such as basil.
Growing in pots
Upright pots, compared to bags on the ground, offer less habitat to slugs and snails. If made of clay, they need more frequent watering because of the surface area exposed to air.
You can grow almost any vegetable in a 30 cm / 12 in pot, for example, but the larger a plant grows, the more you need to feed and water it, especially water.
- Compost used for growing vegetables in containers for, say, two years, is then good to scatter as mulch in the garden, including on beds for vegetables. Then use new compost in the container.
Growing upwards
Using vertical height is a way of increasing horizontal space on the ground, when you can enable growth of roots. For example, tall peas and climbing beans don’t look as though they are using much space at ground level, but their roots spread into a wide area.
Likewise, a hedge near your garden will be rooting a long way from where it grows, usually as far out as the hedge (or tree) is tall – something to ponder when deciding where to grow, and an advantage of container growing when tall woody plants are nearby.
- Squashes and gourds can be grown upwards on a trellis, but their roots are fantastically hungry for food and moisture so this works best for growing in soil, not containers.
- There are courgette varieties bred for vertical growth; they need strong support plus regular pruning of older leaves.
- Dwarf varieties of pole beans, such as Hestia, are good for containers.
If you have space for a cucumber plant to trail, try Iznik F1 or equivalent. Small-fruited cucumber plants are easier to grow than ones making long fruits, and, as in the case of the cucumbers below, it meant less weight to support. I tended the growing stem loosely to the horizontal beam.
Growing under cover
For those of you with under cover space, it’s great for growing in pots, especially when you add shelving to make more of the vertical space. Cherry tomatoes that trail give easy-to-pick fruit from just one plant and with no risk of late blight, as long as (after July), you water the pot’s compost, and not the tomato leaves.
Chilli plants as perennials
Many plants that we grow as annuals from seed can be grown as perennials. To do this you need to prune carefully in late autumn after final harvests, then keep plants healthy overwinter, in a semi-dormant state until growth resumes in spring.
- Prune as hard as you dare, still to green and not older wood, meaning you always leave some of the previous year’s new stems.
- Also water as little as you dare through winter, when plants are in a kind of hibernation.
Aphids can be troublesome on new leaves in early spring. If I see a lot, I take plants outside and squirt the leaves with water. Aphids decrease by early summer and their predators arrive; avoid poisons, even neem oil, which can kill predators.
Stevia rebaudiana
Stevia is easy to grow once you have a seedling, though it’s not easy to buy viable seeds and young seedlings are fragile. Soon, however, they become sturdy bushes, with woody stems by late summer.
The leaves after drying are 300 times sweeter than sugar, but useless for baking because they have no stickiness: the sweetness is from glycosides. Also, the leaves have a strong aftertaste which lingers long on the tongue.
I experimented with overwintering a plant which had grown in soil, by transferring it to a 15 cm / 6 in pot. It was deep, to accommodate the long roots that I had removed in autumn. I then planted this pot back in a greenhouse bed by early May. That autumn I repeated the process for another winter, and it worked well.
Feeding plants in containers
Most vegetables growing in containers quickly use up most of the available nutrients, therefore they often need feeding after, say, two months of growth. In contrast, the same vegetables growing in soil rarely need feeding, and salads in containers, during several months of cropping, need little feeding compared to many vegetables.
There are proprietary organic feeds available and you can also make a feed from nettle and comfrey leaves. I suggest that you use small amounts of feeds to start with, then observe new growth. You want to see leaves growing darker and larger, and plant health staying good.
